Hydraulic vehicle brake systems are known that are in the form of power-assisted brake systems, and besides a muscle-powered master brake cylinder to which the wheel brakes are hydraulically connected and that provides the pressure and volume for actuating wheel brakes, comprise a further electrically controlled pressure and volume providing device, which actuates the wheel brakes in a “brake-by-wire” mode. In the event of a failure of the electrically controlled pressure and volume providing device, a hydraulic fallback level carries out actuation of the wheel brakes using only the muscle power of the driver of the vehicle.
An electrohydraulic brake system is known from DE 195 48 207 A1, which is incorporated by reference in which the driver of the vehicle has no possibility of a direct mechanical-hydraulic actuation of the wheel brakes. A demand for braking by the driver of the vehicle is passed on exclusively electrically (“by-wire”). The brake system comprises a brake pedal, a wheel brake pressure target value generator coupled to the brake pedal, a control unit, an electrically controlled pressure source comprising a pump driven by an electric motor and a hydraulic reservoir that can be charged by the pump, a pressure medium reservoir container and a first and a second valve per hydraulically actuated wheel brake for setting wheel brake pressures. The pressure source is implemented with one circuit, i.e. all wheel brakes are connected to the same pressure source or to the same pressure output of the pressure source. The wheel brakes are exclusively subjected to pressure by the pressure source during the operation of the brake system. In the case of a single leak in the pressure source, a pressure build-up can no longer be carried out in the pressure source and braking is no longer possible.
An electrohydraulic brake system with a pressure source for the front wheel brakes and a further pressure source for the rear wheel brakes is known from DE 100 36 287 A1, which is incorporated by reference wherein each pressure source comprises a hydraulic pump and a high pressure reservoir. Such a brake system is expensive owing to the two pressure sources and requires more installation space than a brake system with only a single pressure source.